According to Hogg, one or the other of Gustav and Dora (yes, there were two) were used at Sevastopol, Stalingrad, and Leningrad, and probably not at Warsaw (more likely the slightly-less-insane 60cm siege howitzers). Gustav (which was the gun at Sevastopol, not Dora) fired maybe 300 shots total before the barrel had to be relined.
Also required an hour to reload, and a quadruple-line rail spur to be built as a firing emplacement. Each, in resources and manpower, cost about as much as two Tiger tank companies. Amazing piece of technology, but a seriously stupid weapon.